Immunity against tetanus in an urban population: gaps of immunity among elderly persons. In a sero-epidemiological survey we investigated the immunity against tetanustoxoid in an urban population. Children and adolescents born in Germany (n = 193) showed an excellent immunity, negative findings being the exception. Among soldiers of the army (n = 136), negative findings were rare too. In these both groups the antibody titiers averaged 4,5 IU/ml. It is generally agreed that an antibody titer of 0,01 IU/ml protects against tetanus. Adults born in Germany (n = 295) had a good immunity: 84% showed antibody - titers against tetanus. Among elderly persons, however, above 60 years of age (n = 1576) there was a striking incidence of negative findings: in 1043 persons (65%) no antibodies could be detected. The remaining 533 persons with positive findings (35%) had antibody-titers in the lower range (average 0,482 IU/ml). Also among foreigners and immigrant labourers from mediterrean countries (Greece, Turkey, Yugoslavia) the frequency of negative findings was high: 183 persons were examined, 70 (39%) were negative 113 (61%) were positive. The incidence of negative findings was highest among women from the mediterrean countries. Several small studies published in Switzerland show a similar tendency. Our paper will encourage further seroepidemiological studies.